This invention relates to signal transmitters for transmitting pressure signals within a flowing liquid, and is more particularly, but no exclusively, concerned with a down-hole transmitter for generating mud pulses in a so-called mud-pulse telemetry system.
It is well known to transmit measurement data from a measuring instrument at the end of a drill string within a borehole by generating pressure variations within the mud flow passing along the drill string and to retrieve the transmitted data by sensing such pressure variations at the surface. The data is transmitted in serial form using some mechanical means of modifying the mud flow in order to produce the necessary pressure variations. These mechanical devices require relatively high forces to operate them with the result that most commercial mud-pulse telemetry systems use either a hydraulic power source or a high power battery power source. The electrical power requirement of the measuring instrumentation is normally only a small fraction of that of the mechanical devices.
Furthermore most existing commercial mud-pulse telemetry systems make use of a mud throttle or valve located within a mechanical assembly attached in some way to the drill collar at the end of the drill string. The mud-pulse transmitter and the drill collar constitute an integrated flow system which must usually be assembled prior to lowering of the drill string in the borehole. For this reason most mud-pulse transmitters are not retrievable or replaceable without withdrawing the whole drill string from the borehole.
It is an object of the invention to provide an improved signal transmitter for transmitting pressure signals within a flowing liquid which provides particular advantages when used in a mud-pulse telemetry system.